Enhancing the Initial Whiteness and Long-Term Thermal Stability of Polyvinyl Chloride by Utilizing Layered Double Hydroxides with Low Surface Basicity

This study investigated the impact of surface basicity on the performance of layered double hydroxides (LDHs) as heat stabilizers for polyvinyl chloride (PVC). LDHs with varying surface basicity were synthesized and characterized using XRD, SEM, BET, and CO2-TPD. The LDHs were then combined with zinc stearate and dibenzoylmethane to create an environmentally friendly heat stabilizer and added to PVC. The resulting PVC composites were evaluated for thermal stability using the oven-aging method. The results showed that a lower Mg/Al molar ratio (2.0) improved the initial whiteness and long-term thermal stability of PVC composites compared to higher ratios (2.5, 3.0, and 3.5). Replacing Mg with Zn in the LDHs had a similar effect to that of reducing the Mg/Al ratio. Crosslinking the laminae of LDHs with 5% silane coupling agent KH-560 reduced the surface basicity of LDHs by 79%, increasing the chromaticity index, b*, and thermal stability time of PVC composites by 48% and 14%, respectively. A descriptive relationship was established between the structure and surface basicity of LDHs and the initial whiteness and long-term thermal stability of PVC composites.


Introduction
Polyvinyl chloride (PVC) is a versatile polymer with high strength, corrosion resistance, durability, good insulation properties, and low-cost. These properties have led to it wide use in a variety of applications, including building materials, packaging, furniture, decorative items, electronics, automotive parts, and medical equipment [1,2]. Despite its advantages, PVC is known for its poor thermal stability, which can cause it to decompose and release hydrogen chloride gas (HCl) at temperatures as low as 90 • C, leading to a decrease in its mechanical and electrical properties. To address this issue, thermal stabilizers are added to PVC during processing to prevent or slow down its decomposition [3][4][5][6][7]. However, the traditional thermal stabilizers, such as lead/cadmium salts and organotin, pose health and environmental hazards, leading to regulations such as EU 76/769/EEC, WEEE, and RoHS that require the use of non-toxic, environmentally friendly, and efficient thermal stabilizers [8]. Thermal stabilizers of β-diketone compounds, zinc stearate, and LDHs show great potential as practical alternatives [4,[9][10][11][12][13][14].

Preparation of LDHs
Preparation of MgAl-CO 3 LDHs: First, 17.20 g Mg(NO 3 ) 2 ·6H 2 O and 12.58 g Al(NO 3 ) 3 · 9H 2 O were added into 80 mL distilled water to form solution A to give n(M 2+ /M 3+ ) = 2, n(OH − )/(2n(M 2+ ) + 3n(M 3+ )) = 1.0, and n(HCO 3 2− )/n(M 3+ ) = 0.5. Then, 9.39 g NaOH was dissolved in 60 mL of distilled water to form solution B, and 1.40 g NaHCO 3 was dissolved in 20 mL distilled water to form solution C. Furthermore, solution A was placed into a 250 mL flask, and solution B was added drop by drop to the flask under stirring at 80 • C until finished and then continuously stirred for 10 min. Then, solution C was added drop by drop to the flask. The obtained slurry was crystallized at 80 • C for 12 h, followed by suction filtration, washing, drying, and grinding to obtain Mg/Al hydrotalcite (labeled as MgAl2.0). The above procedures were repeated for n(M 2+ /M 3+ ) = 2.5, 3.0, and 3.5 to produce Mg/Al hydrotalcite with different Mg/Al molar ratios (labeled as MgAl2.5, MgAl3.0, and MgAl3.5, respectively).
Preparation of cross-linked Mg-Al LDHs: The method of preparing Mg-Al-CO 3 LDHs was followed. Specifically, 0.32 g (5 wt % Mg-Al LDHs) of silane coupling agent KH-560 was added to solution A, then solutions B and C were added drop by drop, and the slurry was stirred at 160 • C for 12 h to obtain cross-linked Mg-Al LDHs, labeled as CL-MgAl2.0.
The surface basicity of LDHs was determined by an automatic chemical adsorption instrument (ChemBET Pulsar TPR/TPD, Quantachrome Instruments, Boynton Beach, FL, USA) by weighing 60 ± 2 mg of samples, purging with helium at 150 • C for 30 min, lowering the temperature to 60 • C, allowing the adsorption of CO 2 gas for 30 min, and then purging the CO 2 on the surface of the sample by helium gas. Desorption was carried out by using helium as the carrier gas at a flow rate is 80 mL/min and a heating rate of 10 • C/min.
The morphology of the LDHs was observed using a field emission scanning electron microscope (ZEISS SUPRA 55, ZEISS, Jena, Germany) with a voltage of 5 kV.
An automatic multifunctional gas adsorption instrument (ASAP 2020, Micromeritics Instrument Corporation, Norcross, GA, USA) was used to determine the specific surface area of LDHs. The samples were vacuum-dried at 150 • C for 2 h, and a nitrogen adsorption isotherm at 77 K was used to calculate the BET-specific surface area.
The particle size and size distribution of the LDHs were measured using the HORIBA LA-960 laser particle size analyzer with air as the dispersant, and the pressure was set at 0.4 MPa.

Preparation of PVC Composites
As shown in Figure 1, 100.0 g of PVC, 50.0 g of DOTP, 30.0 g of CaCO 3 , and 3.0 g of Zn-based thermal stabilizer composed of 1.0 g of TS-Zn and 2.0 g of LDHs were thoroughly mixed and blended in a double-roller mixer that was heated to 170 • C. The resulting PVC composites were compression molded to a 0.8-mm thick film.

Thermal Stability Tests of PVC Composites
Oven-aging tests on the PVC composite samples were performed at 200 • C following the Chinese National Standard Testing Method GB/T 9349-2002 [31].
Color measurements of the PVC composites were preformed using a high-quality portable computer colorimeter (NH310, Shenzhen Threenh Technology Co., Ltd., Shenzhen, China) with the following specifications: light source D65, test aperture Φ8 mm, and display mode CIE L*a*b*C*H*. The lightness index, L*, indicates black and white between Polymers 2023, 15, 1043 4 of 14 0 and 100. The chromaticity index, a*, represents the red-green axis, with positive values indicating red and negative values indicating green. The chromaticity index, b*, represents the yellow-blue axis, with positive values indicating yellow and negative values indicating blue. A yellowish shade means the discoloration of the PVC sample during the initial heating stage, and the change in b* can be used to characterize its whiteness. The greater the b* value, the darker the yellow shade.

Thermal Stability Tests of PVC Composites
Oven-aging tests on the PVC composite samples were performed at 200 °C following the Chinese National Standard Testing Method GB/T 9349-2002 [31].
Color measurements of the PVC composites were preformed using a high-quality portable computer colorimeter (NH310) with the following specifications: light source D65, test aperture Φ8 mm, and display mode CIE L*a*b*C*H*. The lightness index, L*, indicates black and white between 0 and 100. The chromaticity index, a*, represents the red-green axis, with positive values indicating red and negative values indicating green. The chromaticity index, b*, represents the yellow-blue axis, with positive values indicating yellow and negative values indicating blue. A yellowish shade means the discoloration of the PVC sample during the initial heating stage, and the change in b* can be used to characterize its whiteness. The greater the b* value, the darker the yellow shade.
The morphology of the PVC composites before and after aging was observed using a ZEISS Sigma 300, and the attenuated total reflection infrared spectra of the PVC composites before and after aging were measured using a Shimadzu IRTracer-100.

Effect of Metal Species in the Laminae on the Surface Basicity of LDHs and the Thermal Stability of PVC Composites
As demonstrated in Figure 2, when the metal elements in the layers were Mg and Al, strong diffraction peaks were observed at 2θ angles of 11.61°, 23.39°, 34.50°, 38.69°, 60.70°, and 62.05°, which correspond with those of PDF#89-0460, indicating the successful synthesis of an Mg-Al hydrotalcite. After the Mg in the hydrotalcite layers was fully substituted with Ca or Zn, the d(003) diffraction peak shifted to larger angles of 11.69° and 11.71°, respectively, which is consistent with PDF#87-0493 and PDF#48-1023, signifying the preparation of Ca-Al or Zn-Al LDHs. When the ratios of n(Mg)/n(Zn)/n(Al) were 1:1:1, the resulting product displayed characteristic diffraction peaks of typical LDHs, and the d(003) diffraction peak was between that of Mg-Al and Zn-Al LDHs. Similarly, for the ratios of n(Mg)/n(Ca)/n(Al) = 1:1:1, the d(003) diffraction peak of the product was located between those of Mg-Al and Ca-Al LDHs. XRD patterns confirmed the formation of Zn-Mg-Al and Ca-Mg-Al LDHs [32].
The layer charge density and the size of the interlayer anions play a role in determining the interlayer spacing in LDHs. A higher layer positive charge density results in a larger interlayer spacing. This relationship can be observed through the d(003) diffraction peak position in LDHs, with the order of MgAl2.0 > CaMgAl11 > ZnMgAl11 > CaAl2.0 > ZnAl2.0. The morphology of the PVC composites before and after aging was observed using a ZEISS Sigma 300 (ZEISS, Jena, Germany), and the attenuated total reflection infrared spectra of the PVC composites before and after aging were measured using a Shimadzu IRTracer-100 (Shimadzu Corporation, Kyoto, Japan).

Effect of Metal Species in the Laminae on the Surface Basicity of LDHs and the Thermal Stability of PVC Composites
As demonstrated in Figure 2, when the metal elements in the layers were Mg and Al, strong diffraction peaks were observed at 2θ angles of 11.61 • , 23.39 • , 34.50 • , 38.69 • , 60.70 • , and 62.05 • , which correspond with those of PDF#89-0460, indicating the successful synthesis of an Mg-Al hydrotalcite. After the Mg in the hydrotalcite layers was fully substituted with Ca or Zn, the d(003) diffraction peak shifted to larger angles of 11.69 • and 11.71 • , respectively, which is consistent with PDF#87-0493 and PDF#48-1023, signifying the preparation of Ca-Al or Zn-Al LDHs. When the ratios of n(Mg)/n(Zn)/n(Al) were 1:1:1, the resulting product displayed characteristic diffraction peaks of typical LDHs, and the d(003) diffraction peak was between that of Mg-Al and Zn-Al LDHs. Similarly, for the ratios of n(Mg)/n(Ca)/n(Al) = 1:1:1, the d(003) diffraction peak of the product was located between those of Mg-Al and Ca-Al LDHs. XRD patterns confirmed the formation of Zn-Mg-Al and Ca-Mg-Al LDHs [32]. Temperature-programmed desorption (TPD) is a crucial technique used to study the basicity of material surfaces. A higher temperature for the CO2 desorption peak suggests stronger basicity, and a larger area of the desorption peak indicates a greater amount of basicity [33]. Weak basicity (<200 °C) of LDHs was mainly attributed to the hydroxyl  The layer charge density and the size of the interlayer anions play a role in determining the interlayer spacing in LDHs. A higher layer positive charge density results in a larger interlayer spacing. This relationship can be observed through the d(003) diffraction peak position in LDHs, with the order of MgAl2.0 > CaMgAl11 > ZnMgAl11 > CaAl2.0 > ZnAl2.0.
Temperature-programmed desorption (TPD) is a crucial technique used to study the basicity of material surfaces. A higher temperature for the CO 2 desorption peak suggests stronger basicity, and a larger area of the desorption peak indicates a greater amount of basicity [33]. Weak basicity (<200 • C) of LDHs was mainly attributed to the hydroxyl group, while medium to strong basicity (200-300 • C) was due to M-O (where M is a metal element) [24]. The CO 2 -TPD diagrams ( Figure 3) and the derived desorption peak temperature and area (Table 1)   Temperature-programmed desorption (TPD) is a crucial technique used to study the basicity of material surfaces. A higher temperature for the CO2 desorption peak suggests stronger basicity, and a larger area of the desorption peak indicates a greater amount of basicity [33]. Weak basicity (<200 °C) of LDHs was mainly attributed to the hydroxyl group, while medium to strong basicity (200-300 °C) was due to M-O (where M is a metal element) [24]. The CO2-TPD diagrams ( Figure 3) and the derived desorption peak temperature and area (Table 1)     The oven-aging method was employed to evaluate the impact of LDHs on the thermal stability of PVC composites (as shown in Figure 4). This method involves the thermal decomposition of PVC, which results in the release of HCl from the PVC molecular chain and the formation of a conjugated polyene sequence (CPS). As the CPS increases, the color

The Effect of Mg/Al Ratio in Laminae on the Surface Basicity of LDHs and the Thermal Stability of PVC Composites
For the investigation of the relationship between the surface basic strength of hydroxyl groups and the surface basicity of LDHs, a series of Mg-Al LDHs were produced by varying the molar ratio of Mg/Al while keeping the metal elements in the laminae constant ( Figure 5). All of the synthesized products displayed the characteristic diffraction peaks of hydrotalcite. As the molar ratio increased from 2.0 to 3.5, the d(003) characteristic diffraction peak of the LDHs gradually shifted to a smaller angle (2θ were 11.61°, 11.54°, 11.40°, and 11.25°, respectively), which is line with a previous literature report [27]. As shown in Figure 6 and Table 2, the desorption peak temperature decreased with the increase in the Mg/Al molar ratio from 2.0 to 3.5. In contrast, the peak area increased, indicating that while the strength of the basicity of LDHs declined, their surface basicity increased. The main hydroxyl groups of LDHs were Mg3OH and Mg2AlOH, with the former being higher in basicity than the latter. The proportions of Mg3OH were 3%, 20%, and 38% when the Mg/Al molar ratio was 2.0, 3.0, and 4.0, respectively [29]. This suggests that the content of Mg3OH in LDHs increased in the order of MgAl2.0 < MgAl2.5 < MgAl3.0 < MgAl3.5. This order agrees with the surface basicity order of LDHs (MgAl2.0 < MgAl2.5 In the early stage (from 0 min to the color turning yellow) of PVC aging at 200 • C, the chromaticity index, b*, can indicate the degree of yellowing. The smaller b* is, the further the color from yellow [34]. For PVC composites with 1.0 g of stabilizer (TS-Zn) only, at an aging time of 0 min, the chromaticity index b* was 4.08, indicating an excellent initial whiteness of PVC. Adding CaAl2.0, CaMgAl11, ZnMgAl11, and ZnAl2.0 to PVC changed the b* to 7.14, 6.73, 6.53, and 6.38, respectively. In other words, the PVC color became lighter in the order of thermal stabilizers CaAl2.0, CaMgAl11, ZnMgAl11, and ZnAl2.0. Thus, the ability of LDHs to improve the initial whiteness decreased in the order of ZnAl2.0 > ZnMgAl11 > CaMgAl11 > CaAl2.0. This order contradicts the one of positive layer charge density of LDHs (MgAl2.0 > CaMgAl11 > ZnMgAl11 > CaAl2.0 > ZnAl2.0), indicating that the positive layer charge density of LDHs is not fully correlated with the initial whiteness of PVC. In contrast, the ability of LDHs to improve the initial whiteness agrees with the decreasing order of basic strength ZnAl2.0 > ZnMgAl11 > CaMgAl11 > CaAl2.0. Based on these findings, it can be concluded that increasing the basic strength of LDHs leads to an improved initial whiteness of PVC composites.
At 60 min of aging, the PVC with TS-Zn stabilizer turned black, a phenomenon known as "zinc burning." This was due to the absorption of HCl released from PVC decomposition by zinc stearate in TS-Zn, leading to the formation of ZnCl2, which catalytically accelerated PVC decomposition [12]. PVC composites with MgAl2.0/TS-Zn had an initial chromaticity index b* of 7.62 and, although its initial whiteness was not as good as PVC stabilized with TS-Zn only, its color did not turn yellow until 60 min of aging and did not turn brown until 105 min, suggesting that MgAl2.0 improved the long-term thermal stability of PVC. Similarly, PVC with CaAl2.0, CaMgAl11, ZnMgAl11, and ZnAl2.0 changed from yellow to brown after 75, 80, 90, and 105 min of aging, respectively, indicating that the order of ZnAl2.0 = MgAl2.0 > ZnMgAl11 > CaMgAl11 > CaAl2.0 in terms of enhancing the long-term thermal stability of PVC. This order does not match the basic strength order of ZnAl2.0 > ZnMgAl11 > MgAl2.0 > CaMgAl11 > CaAl2.0 or the surface basicity order of MgAl2.0 > ZnAl2.0 > ZnMgAl11 > CaAl2.0 > CaMgAl11. This discrepancy could be due to the absorption of HCl by LDHs in the late stage of aging, leading to the formation of ZnCl 2 , MgCl 2 , and CaCl 2 , which have differing catalytic effects on PVC degradation in the order of ZnCl 2 > CaCl 2 > MgCl 2 . As reported by Ye et al. [10], the long-term thermal stability of PVC is also influenced by chloride species and their contents.

The Effect of Mg/Al Ratio in Laminae on the Surface Basicity of LDHs and the Thermal Stability of PVC Composites
For the investigation of the relationship between the surface basic strength of hydroxyl groups and the surface basicity of LDHs, a series of Mg-Al LDHs were produced by varying the molar ratio of Mg/Al while keeping the metal elements in the laminae constant ( Figure 5). All of the synthesized products displayed the characteristic diffraction peaks of hydrotalcite. As the molar ratio increased from 2.0 to 3.5, the d(003) characteristic diffraction peak of the LDHs gradually shifted to a smaller angle (2θ were 11.61 • , 11.54 • , 11.40 • , and 11.25 • , respectively), which is line with a previous literature report [27].

The Effect of Mg/Al Ratio in Laminae on the Surface Basicity of LDHs and the Thermal Stability of PVC Composites
For the investigation of the relationship between the surface basic strength of hydroxyl groups and the surface basicity of LDHs, a series of Mg-Al LDHs were produced by varying the molar ratio of Mg/Al while keeping the metal elements in the laminae constant ( Figure 5). All of the synthesized products displayed the characteristic diffraction peaks of hydrotalcite. As the molar ratio increased from 2.0 to 3.5, the d(003) characteristic diffraction peak of the LDHs gradually shifted to a smaller angle (2θ were 11.61°, 11.54°, 11.40°, and 11.25°, respectively), which is line with a previous literature report [27]. As shown in Figure 6 and Table 2, the desorption peak temperature decreased with the increase in the Mg/Al molar ratio from 2.0 to 3.5. In contrast, the peak area increased, indicating that while the strength of the basicity of LDHs declined, their surface basicity increased. The main hydroxyl groups of LDHs were Mg3OH and Mg2AlOH, with the former being higher in basicity than the latter. The proportions of Mg3OH were 3%, 20%, and 38% when the Mg/Al molar ratio was 2.0, 3.0, and 4.0, respectively [29]. This suggests that the content of Mg3OH in LDHs increased in the order of MgAl2.0 < MgAl2.5 < MgAl3.0 < MgAl3.5. This order agrees with the surface basicity order of LDHs (MgAl2.0 < MgAl2.5 As shown in Figure 6 and Table 2, the desorption peak temperature decreased with the increase in the Mg/Al molar ratio from 2.0 to 3.5. In contrast, the peak area increased, indicating that while the strength of the basicity of LDHs declined, their surface basicity increased. The main hydroxyl groups of LDHs were Mg 3 OH and Mg 2 AlOH, with the former being higher in basicity than the latter. The proportions of Mg 3 OH were 3%, 20%, and 38% when the Mg/Al molar ratio was 2.0, 3.0, and 4.0, respectively [29]. This suggests that the content of Mg 3 OH in LDHs increased in the order of MgAl2.0 < MgAl2.5 < MgAl3.0 < MgAl3.5. This order agrees with the surface basicity order of LDHs (MgAl2.0 < MgAl2.5 < MgAl3.0 < MgAl3.5), implying that the surface basicity of LDHs is dependent on the Mg 3 OH groups. The thermal stability of PVC composites containing LDHs with different Mg/Al molar ratios was evaluated (Figure 7). The chromaticity index, b*, of PVC composites increased from 7.62 for MgAl2.0 to 8.95 for MgAl3.5 at 0 min aging, indicating that the composites became increasingly yellow as the Mg/Al ratio increased. This reduction in the initial whiteness of PVC composites was further evidenced by the decrease in the time required for the composites to turn yellow, from 90 min for MgAl2.0 to 60 min for MgAl3.5. The time needed for the composites to turn brown also decreased, from 105 min for MgAl2.0 to 75 min for MgAl3.5. These results suggest that as the Mg/Al molar ratio of LDHs increases, the long-term thermal stability of PVC composites deteriorates. This may be due to the increased Mg/Al ratio, making it easier for the PVC molecular chain to be attacked, releasing HCl and forming a conjugated polyene sequence, which reduces the stability and accelerates the consumption of thermal stabilizers. The initial whiteness and long-term thermal stability of PVC composites was dependent on the Mg/Al molar ratio of the LDHs, with smaller ratios resulting in whiter and more stable composites (MgAl2.0 > MgAl2.5 > MgAl3.0 > MgAl3.5). This order is consistent with the strength sequence of basicity (MgAl2.0 > MgAl2.5 > MgAl3.0 > MgAl3.5) but opposite to the surface basicity order (MgAl2.0 < MgAl2.5 < MgAl3.0 < MgAl3.5), meaning that increasing the strength of basicity and reducing the surface basicity of LDHs leads to whiter and more thermally stable PVC.   The thermal stability of PVC composites containing LDHs with different Mg/Al molar ratios was evaluated (Figure 7). The chromaticity index, b*, of PVC composites increased from 7.62 for MgAl2.0 to 8.95 for MgAl3.5 at 0 min aging, indicating that the composites became increasingly yellow as the Mg/Al ratio increased. This reduction in the initial whiteness of PVC composites was further evidenced by the decrease in the time required for the composites to turn yellow, from 90 min for MgAl2.0 to 60 min for MgAl3.5. The time needed for the composites to turn brown also decreased, from 105 min for MgAl2.0 to 75 min for MgAl3.5. These results suggest that as the Mg/Al molar ratio of LDHs increases, the long-term thermal stability of PVC composites deteriorates. This may be due to the increased Mg/Al ratio, making it easier for the PVC molecular chain to be attacked, releasing HCl and forming a conjugated polyene sequence, which reduces the stability and accelerates the consumption of thermal stabilizers. The initial whiteness and long-term thermal stability of PVC composites was dependent on the Mg/Al molar ratio of the LDHs, with smaller ratios resulting in whiter and more stable composites (MgAl2.0 > MgAl2.5 > MgAl3.0 > MgAl3.5). This order is consistent with the strength sequence of basicity (MgAl2.0 > MgAl2.5 > MgAl3.0 > MgAl3.5)but opposite to the surface basicity order (MgAl2.0 < MgAl2.5 < MgAl3.0 < MgAl3.5), meaning that increasing the strength of basicity and reducing the surface basicity of LDHs leads to whiter and more thermally stable PVC.

The Impact of Crosslinking on the Surface Basicity of LDHs and Thermal Stability of PVC Composites
The surface basicity of LDHs is influenced not only by the types and contents of hydroxyl groups but also by crosslinking between the layers. To assess the relationship between the structure and surface basicity, the silane coupling agent KH560 was used to react with the hydroxyl groups on the layers while keeping the metal species and their ratios (Mg/Al = 2.0) constant. As shown in Figure 8, the crosslinked product (CL-MgAl2.0) also had the characteristic diffraction peaks of LDHs. Their positions were similar to MgAl2.0, indicating the interlayer spacings of both MgAl2.0 and CL-MgAl2.0 were consistent, and the layer charge density and interlayer anions were also the same. However, KH560 affected the surface of the LDHs.

The Impact of Crosslinking on the Surface Basicity of LDHs and Thermal Stability of PVC Composites
The surface basicity of LDHs is influenced not only by the types and contents of hydroxyl groups but also by crosslinking between the layers. To assess the relationship between the structure and surface basicity, the silane coupling agent KH560 was used to react with the hydroxyl groups on the layers while keeping the metal species and their ratios (Mg/Al = 2.0) constant. As shown in Figure 8 The surface basicity of LDHs is influenced not only by the types and contents of hydroxyl groups but also by crosslinking between the layers. To assess the relationship between the structure and surface basicity, the silane coupling agent KH560 was used to react with the hydroxyl groups on the layers while keeping the metal species and their ratios (Mg/Al = 2.0) constant. As shown in Figure 8, the crosslinked product (CL-MgAl2.0) also had the characteristic diffraction peaks of LDHs. Their positions were similar to MgAl2.0, indicating the interlayer spacings of both MgAl2.0 and CL-MgAl2.0 were consistent, and the layer charge density and interlayer anions were also the same. However, KH560 affected the surface of the LDHs. The desorption peak temperature of MgAl2.0 was 20°C lower than that of crosslinked CL-MgAl2.0, while the peak area of MgAl2.0 was much larger than that of its crosslinked counterpart ( Figure 9 and Table 3). This suggests that crosslinking has increased the strength of the basicity and reduced the surface basicity of the LDHs. The desorption peak temperature of MgAl2.0 was 20 • C lower than that of crosslinked CL-MgAl2.0, while the peak area of MgAl2.0 was much larger than that of its crosslinked counterpart ( Figure 9 and Table 3). This suggests that crosslinking has increased the strength of the basicity and reduced the surface basicity of the LDHs.   The SEM images of MgAl2.0 ( Figure 10) depict randomly stacked laminae with distinct boundaries. Conversely, the laminae of crosslinked CL-MgAl2.0 were arranged in a more orderly manner, with "stitching" and "stacking" taking place among the laminae, leading to the formation of broader and thicker laminae, which indicates a clear crosslinking between the laminae and supports the results reported in previous works [35]. This may be due to the hydrolysis and condensation of some ethoxy groups of the silane coupling agent in the alkaline solution. The uncondensed -SiOH reacted with the more basic hydroxyl groups (Mg3OH) on the laminae of MgAl2.0, leading to a more stable and orderly stacking between the laminae. This hypothesis is supported by the BET-specific sur-  Table 3. Desorption peak temperature and area of MgAl2.0 and its crosslinked counterpart.

LDHs
Temperature/ • C Area The SEM images of MgAl2.0 ( Figure 10) depict randomly stacked laminae with distinct boundaries. Conversely, the laminae of crosslinked CL-MgAl2.0 were arranged in a more orderly manner, with "stitching" and "stacking" taking place among the laminae, leading to the formation of broader and thicker laminae, which indicates a clear crosslinking between the laminae and supports the results reported in previous works [35]. This may be due to the hydrolysis and condensation of some ethoxy groups of the silane coupling agent in the alkaline solution. The uncondensed -SiOH reacted with the more basic hydroxyl groups (Mg 3 OH) on the laminae of MgAl2.0, leading to a more stable and orderly stacking between the laminae. This hypothesis is supported by the BET-specific surface area measurements. MgAl2.0 had a specific surface area of 11.73 ± 0.04 m 2 /g, while the value was only 3.91 ± 0.18 m 2 /g for crosslinked CL-MgAl2.0. Overall, crosslinking had a similar effect to that of reducing the Mg/Al molar ratio in enhancing the basicity strength of LDHs and reducing their surface basicity.

LDHs
Temperature/°C Area MgAl2.0 165 1188 CL-MgAl2.0 185 253 The SEM images of MgAl2.0 ( Figure 10) depict randomly stacked laminae with distinct boundaries. Conversely, the laminae of crosslinked CL-MgAl2.0 were arranged in a more orderly manner, with "stitching" and "stacking" taking place among the laminae, leading to the formation of broader and thicker laminae, which indicates a clear crosslinking between the laminae and supports the results reported in previous works [35]. This may be due to the hydrolysis and condensation of some ethoxy groups of the silane coupling agent in the alkaline solution. The uncondensed -SiOH reacted with the more basic hydroxyl groups (Mg3OH) on the laminae of MgAl2.0, leading to a more stable and orderly stacking between the laminae. This hypothesis is supported by the BET-specific surface area measurements. MgAl2.0 had a specific surface area of 11.73 ± 0.04 m²/g, while the value was only 3.91 ± 0.18 m²/g for crosslinked CL-MgAl2.0. Overall, crosslinking had a similar effect to that of reducing the Mg/Al molar ratio in enhancing the basicity strength of LDHs and reducing their surface basicity.  The particle size and size distribution of LDHs before and after crosslinking were measured using a laser particle size analyzer in the dry method, as shown in Figure 11 and Table 4. The particle size of both MgAl2.0 and CL-MgAl2.0 had a normal distribution, with most particles ranging from 2.0 to 9.5 µm. The average particle size (D0.5) was 4.51 µm for MgAl2.0 and 4.93 µm for CL-MgAl2.0. The results suggest that the particle size and size distribution of LDHs did not change significantly after crosslinking.
Polymers 2023, 15, 1043 11 of 15 The particle size and size distribution of LDHs before and after crosslinking were measured using a laser particle size analyzer in the dry method, as shown in Figure 11 and Table 4. The particle size of both MgAl2.0 and CL-MgAl2.0 had a normal distribution, with most particles ranging from 2.0 to 9.5 μm. The average particle size (D0.5) was 4.51 μm for MgAl2.0 and 4.93 μm for CL-MgAl2.0. The results suggest that the particle size and size distribution of LDHs did not change significantly after crosslinking.  The effects of crosslinking LDHs on the thermal stability of PVC composites are de-  The effects of crosslinking LDHs on the thermal stability of PVC composites are depicted in Figure 12. The composite containing MgAl2.0/TS-Zn had a chromaticity index, b*, of 7.62 after 0 min of aging. It turned yellow after 60 min of aging and became brown after 105 min. On the other hand, the composite with crosslinked MgAl2.0 (CL-MgAl2.0/TS-Zn) had a chromaticity index, b*, of 4.23 after 0 min, and it turned yellow and brown after 90 min and 120 min of aging, respectively. These results indicate that crosslinking improved the initial whiteness of the PVC composite and enhanced its long-term thermal stability by 14.3%. Combining the results from the CO 2 -TPD analysis ( Figure 9 and Table 3), it is evident that crosslinking increased the strength of basicity, reduced surface basicity, improved initial whiteness, and enhanced the long-term thermal stability of PVC composites. The effect of crosslinking was similar to that of lowering the Mg/Al molar ratio in improving the initial whiteness and long-term thermal stability of PVC.  The effects of crosslinking LDHs on the thermal stability of PVC composites are depicted in Figure 12. The composite containing MgAl2.0/TS-Zn had a chromaticity index, b*, of 7.62 after 0 min of aging. It turned yellow after 60 min of aging and became brown after 105 min. On the other hand, the composite with crosslinked MgAl2.0 (CL-MgAl2.0/TS-Zn) had a chromaticity index, b*, of 4.23 after 0 min, and it turned yellow and brown after 90 min and 120 min of aging, respectively. These results indicate that crosslinking improved the initial whiteness of the PVC composite and enhanced its long-term thermal stability by 14.3%. Combining the results from the CO2-TPD analysis ( Figure 9 and Table 3), it is evident that crosslinking increased the strength of basicity, reduced surface basicity, improved initial whiteness, and enhanced the long-term thermal stability of PVC composites. The effect of crosslinking was similar to that of lowering the Mg/Al molar ratio in improving the initial whiteness and long-term thermal stability of PVC. To further investigate the impact of crosslinking on the thermal stability of PVC, SEM and ART-FTIR were performed on the PVC composite materials before and after aging. The outcomes are presented in Figures 13 and 14. At 0 min of aging, the PVC composite materials with MgAl2.0/TS-Zn and CL-MgAl2.0/TS-Zn were smooth and compact, demonstrating successful PVC plasticization. After 105 min of aging, tiny pores emerged in the PVC composite materials containing MgAl2.0/TS-Zn and CL-MgAl2.0/TS-Zn, indicating that the PVC had decomposed. The PVC composite material containing MgAl2.0/TS-Zn showed larger pores, implying that its decomposition was more severe. This outcome aligns with the results of the oven aging process.
As shown in Figure 14, the ART-FTIR of PVC composite materials containing MgAl2.0/ TS-Zn and CL-MgAl2.0/TS-Zn aged at different times have similar absorption peaks. The peaks at 1255, 694, and 616 cm −1 correspond to the C-Cl bonds in PVC, 1427 cm −1 is the -CH 2 -peak in PVC, 874 cm −1 is the benzene ring positional substitution peak in DOTP, 1719 cm −1 is the -C=O peak in DOTP, 1255 and 1123 cm −1 correspond to the -C-O in DOTP, 2959 and 2860 cm −1 are the -CH 3 and -CH 2 -peaks, and 874 cm −1 is the characteristic peak of CaCO 3 . The -OH characteristic peak in LDHs is found between 3130 and 3670 cm −1 .
After aging for 105 min, the 874 cm −1 CaCO 3 peak remained unchanged, suggesting that the filler did not react. However, the intensity of the 1719 cm −1 -C=O peak decreased significantly due to the volatilization of DOTP during high-temperature aging. The C-Cl peaks in PVC, specifically those at 1255, 694, and 616 cm −1 , also decreased significantly, with a more noticeable decrease in PVC composite materials containing MgAl2.0/TS-Zn, indicating that these materials underwent more severe decomposition and released more HCl. The -OH absorption peak in PVC composite materials containing MgAl2.0/TS-Zn nearly disappeared, while it was still visible in PVC composite materials containing CL-MgAl2.0/TS-Zn, which showed that MgAl2.0 was consumed more quickly, and the decomposition in PVC composite materials containing MgAl2.0/TS-Zn was more severe.
To further investigate the impact of crosslinking on the thermal stability of PVC, SEM and ART-FTIR were performed on the PVC composite materials before and after aging. The outcomes are presented in Figures 13 and 14. At 0 min of aging, the PVC composite materials with MgAl2.0/TS-Zn and CL-MgAl2.0/TS-Zn were smooth and compact, demonstrating successful PVC plasticization. After 105 min of aging, tiny pores emerged in the PVC composite materials containing MgAl2.0/TS-Zn and CL-MgAl2.0/TS-Zn, indicating that the PVC had decomposed. The PVC composite material containing MgAl2.0/TS-Zn showed larger pores, implying that its decomposition was more severe. This outcome aligns with the results of the oven aging process. As shown in Figure 14, the ART-FTIR of PVC composite materials containing MgAl2.0/TS-Zn and CL-MgAl2.0/TS-Zn aged at different times have similar absorption peaks. The peaks at 1255, 694, and 616 cm −1 correspond to the C-Cl bonds in PVC, 1427 cm −1 is the -CH2-peak in PVC, 874 cm −1 is the benzene ring positional substitution peak in DOTP, 1719 cm −1 is the -C=O peak in DOTP, 1255 and 1123 cm −1 correspond to the -C-O in DOTP, 2959 and 2860 cm −1 are the -CH3 and -CH2-peaks, and 874 cm −1 is the characteristic peak of CaCO3. The -OH characteristic peak in LDHs is found between 3130 and 3670 cm −1 .
After aging for 105 min, the 874 cm −1 CaCO3 peak remained unchanged, suggesting that the filler did not react. However, the intensity of the 1719 cm −1 -C=O peak decreased significantly due to the volatilization of DOTP during high-temperature aging. The C-Cl peaks in PVC, specifically those at 1255, 694, and 616 cm −1 , also decreased significantly, with a more noticeable decrease in PVC composite materials containing MgAl2.0/TS-Zn, indicating that these materials underwent more severe decomposition and released more HCl. The -OH absorption peak in PVC composite materials containing MgAl2.0/TS-Zn nearly disappeared, while it was still visible in PVC composite materials containing CL-MgAl2.0/TS-Zn, which showed that MgAl2.0 was consumed more quickly, and the decomposition in PVC composite materials containing MgAl2.0/TS-Zn was more severe.

Conclusions
Developing environmentally friendly thermal stabilizers is critical for the successful application of PVC. In this study, LDHs with varying Mg/Al molar ratios and metal spe-

Conclusions
Developing environmentally friendly thermal stabilizers is critical for the successful application of PVC. In this study, LDHs with varying Mg/Al molar ratios and metal species were synthesized and cross-linked to evaluate their impact on PVC properties. The results showed that a lower Mg/Al ratio of 2.0 improved the initial whiteness and long-term thermal stability of PVC-LDH composites compared to higher ratios of 2.5, 3.0, and 3.5. Furthermore, substituting Mg in LDHs with Zn had a similar effect to that of lowering the Mg/Al molar ratio in improving PVC properties. A third approach, crosslinking the LDH laminae with 5% silane coupling agent KH-560, reduced the surface basicity by 79%, improved the initial whiteness of PVC by 48%, and increased its long-term thermal stability by 14%. These three approaches work by reducing the surface basicity of LDHs and enhancing their basic strength, reducing the negative effects of basicity on PVC molecular chains, such as the release of HCl, consumption of thermal stabilizer, and decreased stability of PVC. The weak basicity (<200 • C) of LDHs was primarily attributed to Mg 3 OH, while reducing the molar ratio of divalent and trivalent metal elements in LDHs and increasing the crosslinking degree of LDH layers significantly reduced their surface basicity. A medium to strong basicity (200-300 • C) of LDHs was caused by M-O (where M represents a metal element), and an increase in the content of metal elements with a high electron-donating ability strengthened the surface basicity of LDHs.

Patents
In relation to this research, a patent (ZL201910476975.4) was granted by China National Intellectual Property Administration.